Fix an XMLRPC uploading file issue

[crazytonyli]

Description

Fixes wordpress-mobile/WordPress-iOS#22659

Testing Details

See the added unit tests.

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[mokagio]

Approving to unblock. I checked it out locally and saw that the test fails with the original code.

However, I don't understand how this works 🤔 I've been looking into the WPXMLRPCEncoder method and a bit down the chain and I don't see anything that would result in the second call (however unnecessary) to it returning a different value from the first 🤔

[crazytonyli]

The root cause is at the encodingInputStream function. The first call works as expected with full base64 encoded content. Since the first call reads the stream to its end, the second call start reading from the end, which of course returns nothing, and get an empty base64 encoded content.

[mokagio]

Got it. Thanks for following up @crazytonyli